1. Field of the Invention
The invention relates to a process for the production of a film based on plasticized polyvinyl acetal by embossing and to the use of the films for the production of glass laminates.
2. Description of the Related Art
Laminated safety glass consists usually of two glass panes and one adhesive film which combines the glass panes and is based on plasticized polyvinyl acetal, preferably of polyvinyl butyral (PVB). Laminated safety glass is used in particular as windscreens in motor vehicles, it being possible for a glass pane to be replaced, if necessary, by a polymer pane.
In the construction sector, too, silicate glass/silicate glass composites or silicate glass/polymer composites are used e.g. as window panes or as intermediate walls, multiple composites, i.e. composites consisting of more than two supporting layers being used, if necessary and depending on their application, e.g. as bullet-proof glass.
Plasticiser-containing films based on polyvinyl acetal, in particular on polyvinyl butyral (PVB) for the manufacture of safety composite glass are soft and tacky even at room temperature. Although the high tackiness is essential for holding the composite of glass/film/glass together in composite glass, the tackiness needs to be temporarily eliminated or, however, at least suppressed, for transportation and the process of processing them to such glass. The inherent tackiness of the films can be reduced by a certain roughness.
Moreover, it needs to be possible for the air present between the film and the glass to be removed during processing of the film to form composite safety glass. In this respect, it is generally known to provide the films on one or both sides with a roughened surface. The air enclosed during the manufacture of the glass laminate is able to escape via the roughened surface such that a bubble-free laminate is obtained.
A typical process for the manufacture of films with a roughened surface is known from EP 0 185 863 B1 as a melt fracture process. Melt fracture processes lead to irregularly (stochastically) roughened surfaces.
Embossing processes are further processes described in the state of the art for the production of a roughened surface. The common feature of all film surfaces produced by embossing processes is a regular (non-stochastic) surface structure which exhibits good ventilation behaviour, particularly in the production process for glass laminates by the vacuum bag process, and consequently permits short process times and wide processing windows.
In comparison with melt fracture processes, embossing processes have the advantage that the regular surface structure obtained allows more rapid and simpler air removal during laminate production.
EP 0 741 640 B1 describes such an embossing process for the production of a surface embossed on both sides by means of two embossing rollers by means of which the film is provided with a regular line structure of the saw tooth type. The lines embossed on each side of the film cross each other at an angle of >25° such that a so-called moiré pattern is prevented from forming in the composite glass.
EP 1 233 007 A1 discloses an embossing process for avoiding the moiré effect which process produces a regular embossing structure on each side of the film. To avoid interferences, the line structures of the two film sides have different repetition frequencies.
Another process which is described in U.S. Pat. No. 5,972,280 uses only one roller to emboss the surface structure, instead of two embossing rollers, and a structured steel band fitting snugly to the roller via rolls and compressed air, the film being guided during the embossing process through the gap between the embossing roller and the steel band.
U.S. Pat. No. 4,671,913 discloses a process for embossing PVB films, the film being embossed in a single operating process between two structured rollers. The rollers —and consequently also the embossed film—have a roughness Rz of 10 to 60 μm. US 2003/0022015, WO 01/72509, U.S. Pat. No. 6,077,374 and U.S. 60/934,741 describe single stage and two-stage embossing processes for PVB films by means of embossing rollers of steel and pressing rollers with a rubber coating.
WO 2014021459 A1 discloses the manufacture of a plasticized PVB-film by embossing channels having a wide pitch of at least 750 μm.
For efficiently producing laminated glass the interlayer should have good de-airing properties and fast melt-down of the surface structure.